Electric Heating Assisted Passive and Active Regeneration for Efficient Emission Controls of Diesel Engines

ABSTRACT

A method of heating an exhaust gas in an exhaust aftertreatment system by selecting a heating mode between different heating modes based on an engine load and a status of a component of the exhaust aftertreatment system. The method includes heating the exhaust gas by operating the electric heater in the selected heating mode and heating the exhaust gas to reduce an exhaust temperature gradient across the exhaust conduit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 13/773,176,filed on Feb. 21, 2013, which claims the benefit of 61/601,923, filed onFeb. 22, 2012. The disclosures of the above applications areincorporated herein by reference.

FIELD

The present disclosure relates to exhaust aftertreatment systems fordiesel engines, and more particularly to electric heating and control toprovide assisted heating in the exhaust aftertreatment systems.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure and may notconstitute prior art.

Diesel engines have been used in a variety of applications such aslocomotives, marines and engine-generators. The U.S. EnvironmentalProtection Agency (EPA) and the California Air Resources Board (GARB),as well as other regulatory agencies around the world, impose strictlimitations on the contents of emissions from diesel engines, such asparticulate matter (PM), hydrocarbon (HC) and NOx. Accordingly, exhaustaftertreatment systems have been employed and generally include a DieselOxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and an SCR(Selective Catalytic Reduction of NOx) to treat the exhaust gas and tocontrol emissions to atmosphere or the outside environment.

Various chemical reactions occur in the DOC and SCR to convert harmfulnitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbon(HC) into N₂, CO₂ and water. The DPF is designed to remove dieselparticulate matter (PM) from the exhaust gas. Normally these chemicalreactions would take place at high temperatures. With the use ofcatalysts, the chemical reactions can occur at much lower temperatures.Sufficient energy in the form of heat, however, must still be suppliedto the catalysts to expedite the chemical reactions. Therefore,performance of the exhaust aftertreatment system is highly dependent onthe temperature of the exhaust gas, which carries the desired energy andheat to the catalysts. The normal temperature of the exhaust gas,however, does not always meet requirements for the desired chemicalreactions. When the normal exhaust temperature is lower than the targettemperature, the exhaust aftertreatment system cannot effectively treatthe exhaust gas, resulting in higher emissions to the outsideenvironment.

One method of increasing the exhaust gas temperature is throughinjecting hydrocarbon upstream from a DOC either in the exhaust pipe orinside the cylinder during the exhaust stroke. This method increasesfuel consumption and also changes composition of the exhaust gas. Forexample, when fuel injection is injected in the exhaust, NO₂ generationin the DOC is significantly reduced. NO₂ is an effective reagent forpassive regeneration of DPF at much lower temperature range. Therefore,the reduced NO₂ generation adversely affects the passive regeneration ofthe DPF.

SUMMARY

In one form, a method of heating an exhaust gas in an exhaustaftertreatment system includes an electric heater, a Diesel OxidationCatalyst (DOC) and a diesel particulate filter (DPF). The methodcomprises heating an exhaust gas to a predetermined temperature when theDPF is not actively regenerated, and heating the exhaust gas to reducean exhaust temperature gradient across the exhaust conduit.

In another form, a method of heating an exhaust gas in an exhaustaftertreatment system includes an electric heater, a Diesel OxidationCatalyst (DOC) and a diesel particulate filter (DPF). The methodcomprises selecting a heating mode between a plurality of heating modesbased on engine load and status of the DPF. The method further comprisesheating the exhaust gas by operating the electric heater in the selectedheating mode, and operating the electric heater in a passiveregeneration heating mode to heat an exhaust gas to a predeterminedtemperature to increase NO2 generation when the DPF is not activelyregenerated and the engine load is less than or equal to approximately25%.

In still another form, a method of heating an exhaust gas in an exhaustafter treatment system comprises selecting a heating mode between aplurality of heating modes based on an engine load and a status of acomponent of the exhaust aftertreatment system. The method furthercomprises heating the exhaust gas by operating the electric heater inthe selected heating mode, and operating the electric heater in apassive regeneration heating mode to heat an exhaust gas to apredetermined temperature to increase NO2 generation when an engine loadis less than or equal to approximately 25%.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, incorporated in and forming a part of thespecification, illustrate several aspects of the present invention and,together with the description, serve to explain the principles of theinvention. The components in the figures are not necessarily to scale.In the drawings:

FIG. 1 is a schematic view of an engine system including a heatingmodule constructed in accordance with the teachings of the presentdisclosure;

FIG. 2 is a schematic view of a heating module constructed in accordancewith the teachings of the present disclosure;

FIG. 3 is a graph showing relationship between concentration of NO₂ andcatalyst temperature;

FIG. 4 is a schematic view of an electric heater;

FIG. 5 is a graph showing a heating strategy for operating the electricheater;

FIG. 6 is a table showing the properties of the exhaust gas at differentengine loads; and

FIG. 7 is a schematic view of another form of an engine system includinga heating module constructed in accordance with the teachings of thepresent disclosure.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is in no wayintended to limit the present invention, its application, or uses. Itshould also be understood that steps within a method may be executed indifferent order without altering the principles of the invention.

Referring to FIG. 1, an engine system 10 generally includes a dieselengine 12, a generator 14, a turbocharger 16, and an exhaustaftertreatment system 18. The exhaust aftertreatment system 18 isdisposed downstream from a turbocharger 16 for treating exhaust gasesfrom the diesel engine 12 before the exhaust gases are released toatmosphere. The exhaust aftertreatment system 18 includes a heatingmodule 20, a DOC 22, DPF 24, and an SCR 26. The heating module 20includes an electric heater 28 disposed upstream from the DOC 22, and aheater control module 30 for controlling operation of the electricheater 28. The exhaust aftertreament system 18 includes an upstreamexhaust conduit 32 that receives the electric heater 28 therein, anintermediate exhaust conduit 34 in which the DOC 22 and DPF 24 arereceived, and a downstream exhaust conduit 36 in which the SCR isdisposed.

The DOC 22 is disposed downstream from the electric heater 28 and servesas a catalyst to oxide carbon monoxide and any unburnt hydrocarbons inthe exhaust gas. In addition, The DOC 22 converts harmful nitric oxide(NO) into nitrogen dioxide (NO₂). The DPF 24 is disposed downstream fromthe DOC 22 to remove diesel particulate matter (PM) or soot from theexhaust gas. The SCR 26 is disposed downstream from the DPF 24 and, withthe aid of a catalyst, converts nitrogen oxides (NOx) into nitrogen (N₂)and water. A urea water solution injector 27 is disposed downstream fromthe DPF 24 and upstream from the SCR 26 for injecting urea watersolution into the stream of the exhaust gas. When urea water solution isused as the reductant in the SCR 18, NOx is reduced into N₂, H₂O and CO₂in the following reaction:

4NO+2(NH₂)₂CO+O₂→4N₂+4H₂O+2CO₂

The electric heater 28 provides assisted heating of the exhaust gasflowing in the exhaust conduits 32, 34, 36. The generator 14 isconnected to the diesel engine 12 to drive the diesel engine 12 duringengine startup as an option and to supply electricity to the electricheater 34 during normal engine operation. The heater control module 30strategically controls the electric heater 28 in different heating modesto facilitate both active and passive regeneration of the DPF 24.

Regeneration is the process of burning and removing the accumulatedparticulates matters from the DPF 24. Regeneration can occur passivelyor actively. Passive regeneration can occur in normal engine operationwhen the temperature of the exhaust gas is sufficiently high. Activeregeneration can occur based on a monitored DPF condition or based on apredetermined timing schedule by introducing very high heat to theexhaust aftertreatment system 10. Active regeneration can be achieved byproper engine control management to increase the exhaust temperaturethrough late fuel injection or injection during the expansion stroke.Active regeneration can also be achieved through assisted heating by anelectric heater. Active regeneration requires much more heat thanpassive regeneration and thus subjects the ceramic structure of the DPF24 to the risk of cracking and decreases catalytic coating life time.

Referring to FIG. 2, the heater control module 30 strategically controlsoperation of the electric heater 28 based on an engine load and a statusof the DPF 24 to provide assisted heating in both active and passiveregeneration of the DPF. The heater control module 24 may be a part ofan engine control unit (ECU) (not shown) or external to the ECU. The ECUcontrols operation of the diesel engine 12, a fuel injection system (notshown), among others, and acquires and stores various parametersrelating to engine operating conditions, including but not limited to,exhaust temperature, diesel engine load, flow conditions (air flow andair pressure etc.) The heater control module 30 receives inputs from theECU to make the proper determination how to operate the electric heater28. The control module could also receive information from stand aloneafter treatment control systems.

The heater control module 30 includes a heating mode determinationmodule 62 and a heater operating module 63 including a passiveregeneration heating module 64 and an active regeneration heating module66. The electric heater 22 can be operated in two operating modes:passive regeneration heating mode and active regeneration heating mode.The heating mode determination module 62 determines a desired heatingmode based on an engine load and the status of the DPF 24. When the DPF24 is actively regenerated, the desired heating mode is the activeregeneration heating mode. When the DPF 24 is not actively regeneratedand the engine load is low, for example, at 10%, the desired heatingmode is the passive regeneration heating mode. The heating modedetermination module 62 may include a heating strategy that specifiesthe correlation among the heating modes, duration, engine loads and thedesired exhaust temperature rise. The heating mode determination module62 also determines when the electric heater 28 should be turned on oroff during normal engine operation. In response to the determination ofthe heating mode determination module 62, the heater operating module 63operates the electric heater 28 accordingly.

In the passive regeneration heating mode, the electric heater 28 iscontrolled to heat the exhaust gas to a predetermined temperature whichallows for optimum NO₂ generation in the DOC 22. NO₂ is an effectivereactant for passive regeneration of DPF 24. Increasing NO₂ generationcan facilitate passive regeneration of DPF 24. In the active heatingmode, the electric heater 28 is controlled to heat the exhaust gasdifferently to reduce exhaust temperature gradient across the exhaustconduits. When the temperature gradient is reduced, the activeregeneration can be accomplished more efficiently.

When the heating mode determination module 62 determines that thepassive heating mode is desired, the passive regeneration heating module64 then controls the electric heater 28 to heat the exhaust gas to apredetermined temperature. The passive regeneration heating module 64calculates and determines the desired temperature rise based on anexhaust temperature and the predetermined temperature. The exhausttemperature may be obtained from the input from the ECU, temperaturesensors. The predetermined temperature depends on the properties of thecatalysts in the DOC 14 and is set to allow for optimum NO₂ generation.

Referring to FIG. 3, the NO₂ concentration at the outlet of the DOC 14is dependent on the temperature of the exhaust gas. For a BASF DOCcatalyst, the NO₂ concentration is relatively high when the catalysttemperature is in the range of 300 to 460° C., particularly in the rangefrom 320 to 380° C. Therefore, the predetermined temperature is set tobe in the range of 300 to 460° C., and preferably in the range from 320to 380° C. When the electric heater 28 heats exhaust gas to thepredetermined temperature, an optimum amount of NO₂ is generated tofacilitate passive regeneration of the DPF 24. With the extensivepassive regeneration of DPF, the particulate matter is accumulated onthe DPF at a lower rate, thereby reducing the frequency for activeregeneration. As a result, the likelihood of DPF ceramic cracking anddegradation of the catalysts due to high heat associated with activeregeneration (generally in the range of 500 to 650° C.) are reduced.

Referring back to FIG. 2, when the DPF 24 is actively regenerated, thedesired heating mode is the active regeneration heating mode. The activeregeneration heating module 66 controls the electric heater 28 toprovide differential heating to the exhaust gas. The electrical heater22 generates more heat along the periphery of the electric heater andless heat at the center of the exhaust conduit.

The exhaust conduit generally has a relatively higher temperature alongthe central axis of the conduit and a relatively lower temperatureproximate the conduit wall. To ensure effective active regenerationacross the DPF 24, the exhaust gas proximate the exhaust conduit wallalso needs to be heated to the desired active regeneration temperature.Due to the temperature gradient across the cross section of the exhaustconduit, the exhaust gas proximate the center of the exhaust conduit isunnecessarily overheated, subjecting the center portion of the DPF 24 tohigher heat and higher risk of cracks. By operating the electric heater28 to reduce the temperature gradient, less heat is required to heat theexhaust gas to the desired active regeneration temperature. Therefore,the likelihood of overheating at the center of the DPF and theaccompanying problems is reduced.

Referring to FIG. 4, an exemplary embodiment of the electric heater 28is shown to have a low watt density zone 40 proximate the center and ahigh watt density zone 42 along the periphery of the electric heater 28.The electric heater 28 can provide differential heating across theexhaust conduit.

The electric heater 28 is powered by the generator 14. The generator 14drives the diesel engine 30 during engine startup. After the dieselengine 30 starts to operate on its own, the generator 14 is driven bythe diesel engine 14 to generate electricity to power other electronicsor electrical devices. The heating strategy allows for use of availableelectricity generating capacity when it is not needed to power the otherelectrical and electronic systems during low engine load operation.

Referring to FIG. 5, the heating mode determination module 62 includes aheating strategy which specifies the correlations among the heatingmodes, the exhaust temperature rise, the engine loads. As shown in theexemplary diagram, when the engine load is low and the DPF backpressureis in the range of medium to high, the target exhaust temperature rise(delta) would be low and the electric heater 28 is operated in thepassive regeneration mode. For example, the electric heater 22 is in thepassive regeneration heating mode when the diesel engine 12 is operatingnear low load conditions such as 10% load. The electric heater 22demands less electric power from the generator 14 because the desiredtemperature rise (delta) is less than that for active regeneration andbecause less exhaust gas is generated from the diesel engine 30 due tothe low engine load.

As the engine load continues to increase, for example, from 10% to 25%,to 50%, to 75%, the electric heater 28 is turned off. Activeregeneration of DPF may be initiated when the engine load is low oraccording to a predetermined schedule to benefit from heating lowerexhaust mass flow. When the DPF is actively regenerated, for example, atan engine load of 25%, the electric heater is turned on and operated inthe active regeneration heating mode to provide differential heating.When the active regeneration is completed and the engine load starts toincrease, the electric heater 28 is turned off.

Referring to FIG. 6, the table illustrates the exhaust contents fordifferent load conditions. As shown, when the diesel engine is operatedunder the 10% load condition, the exhaust gas exhibits the lowestexhaust flow (1925 cfm) and the highest available specific NOx (6.8g/bhp-hr) among the 5 load conditions for a gen-set type of large dieselengine. For example, had the exhaust temperature been raised from 235 C(455 F) to a temperature that is within the DOC's NO2 generation sweettemperature window of 320 to 380° C., the DOC downstream of the heaterwill generate maximum amount of NO₂ due to higher available NOx underthis load engine condition. NO₂ passively oxidizes the particulatematter loaded DPF downstream of the DOC at its maximum rate.Additionally, the delta T rise is only 85° C. which will minimize energyconsumption in comparison with an active regeneration which will have adelta T as high as 350° C.

For the 10% load condition on this Gen-set with a flow of 81.6 kg/min,it will require 121 KW energy input to heat the exhaust and have a deltaT rise of 85° C. It will need 450 KW to heat the exhaust up to 550° C.at 25% load condition with a flow of 137.3 kg/min.

For the notch 1 condition on a GE locomotive engine with a flow of 54.8kg/min, it will require 73 KW energy input to heat the exhaust and havea temperature rise (delta) of 76° C. up to 355° C. It will need 315 KWto heat the exhaust up to 607° C. at the same notch 1 condition.

With the extensive passive regeneration, the accumulation of the sootand PMs on the DPF 24, as well as the backpressure of the DPF, arereduced. As a result, the active regeneration periods and frequenciescan be significantly reduced, thereby enhancing durability of theexpensive DPF. The electric heating strategy of the present disclosuremay replace the fuel-injection-based active regeneration.

Referring to FIG. 6, the heating module 20 of the present disclosureapplies to all diesel engines which can generate electricity while inoperation, preferably to those non-EGR diesel engines having highengine-out NOx at lower duty cycles. As shown, the heating module 20 canbe applied to a catalyzed DPF only exhaust system, as well as an exhaustaftertreatment system 50 that includes DOC 52 and DPF 54 without SCR.

The heating module 20 of the present disclosure has at least thefollowing benefits:

1. Utilizing available electricity generating capacity when it is notneeded for other operations on a diesel-generator or a marine engine ora locomotive at low load to assist in passively regenerating the DPF aspart of the engine's emission control system.

2. Reducing the frequency of diesel fuel injection based activeregeneration and hence enhancing fuel economy of the engine operation.

3. Reducing DPF operational soot loading through heating assistedpassive regeneration to minimize overall operational backpressure.

4. Reducing risks of DPF cracking caused by soot overloaded runawayregenerations through heating assisted passive regeneration.

5. Improving exhaust aftertreatment system's performance throughdelivering more uniform exhaust temperatures across the system's inletface.

Additionally, the present disclosure may include methods of heatingportions of the gas flow in a more indirect matter. For example, thesystem could sense cooler portions within the gas flow cross section andprovide heat where needed to provide a more even temperaturedistribution and compensate for heat losses. In addition, for systemsthat require more electricity than is available to regenerate the entiregas stream cross-section, the system may regenerate in certain sectionsor zones at different times. These alternate forms of the presentdisclosure would also have a corresponding heater type that supportszone heating across the cross-section of gas flow, such as, by way ofexample, layered heaters or modular heat trace heaters such as thosedisclosed in pending U.S. application Ser. No. 11/238,747 titled“Modular Layered Heater System” and in U.S. Pat. No. 7,626,146 titled“Modular Heater Systems,” both of which are commonly assigned with thepresent application and the contents of which are incorporated byreference herein in their entirety.

The broad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited sincemodifications will become apparent from the following claims.

What is claimed is:
 1. A method of heating an exhaust gas in an exhaustaftertreatment system including an electric heater, a Diesel OxidationCatalyst (DOC) and a diesel particulate filter (DPF), the methodcomprising: heating an exhaust gas to a predetermined temperature whenthe DPF is not actively regenerated; and heating the exhaust gas toreduce an exhaust temperature gradient across the exhaust conduit. 2.The method according to claim 1, wherein the predetermined temperatureis a function of properties of the DOC.
 3. The method according to claim1, further comprising increasing NO₂ concentration at an outlet of theDOC when the DPF is not actively regenerated.
 4. The method according toclaim 1, wherein the predetermined temperature is in a range from 300 to460° C.
 5. The method according to claim 1, wherein the predeterminedtemperature is in a range from 320 to 380° C.
 6. The method according toclaim 1, further comprising providing more heat proximate a wall of anexhaust conduit and less heat proximate a center of the exhaust conduitto reduce the exhaust temperature gradient.
 7. The method according toclaim 1, further comprising determining an operating state of theelectric heater during normal engine operation.
 8. A method of heatingan exhaust gas in an exhaust aftertreatment system including an electricheater, a Diesel Oxidation Catalyst (DOC) and a diesel particulatefilter (DPF), the method comprising: selecting a heating mode between aplurality of heating modes based on engine load and status of the DPF;heating the exhaust gas by operating the electric heater in the selectedheating mode; and operating the electric heater in a passiveregeneration heating mode to heat an exhaust gas to a predeterminedtemperature to increase NO₂ generation when the DPF is not activelyregenerated and the engine load is less than or equal to approximately25%.
 9. The method according to claim 8, further comprising operatingthe electric heater in an active regeneration heating mode to providedifferential heating when the DPF is actively regenerated.
 10. Themethod according to claim 9, further comprising operating the electricheater to generate more heat proximate a periphery of the heater andless heat proximate a center of the exhaust conduit to reduce an exhausttemperature gradient across an exhaust conduit.
 11. The method accordingto claim 9, further comprising providing more heat proximate a wall ofan exhaust conduit and less heat proximate a center of the exhaustconduit to reduce an exhaust temperature gradient across an exhaustconduit.
 12. The method according to claim 9, wherein the step ofoperating the electric heater in the active regeneration heating modedoes not include injecting fuel into the exhaust gas.
 13. The methodaccording to claim 8, wherein the predetermined temperature is afunction of properties of the DOC.
 14. The method according to claim 8,wherein the predetermined temperature is in a range from 300 to 460° C.15. The method according to claim 8, wherein the predeterminedtemperature is in a range from 320 to 380° C.
 16. The method accordingto claim 8, wherein the step of selecting the heating mode furthercomprises: measuring an exhaust temperature of the exhaust gas; anddetermining a desired temperature rise above the exhaust temperature forthe plurality of heating modes.
 17. The method according to claim 16,wherein the step of selecting the heating mode further comprisesoperating the electric heater in one of the plurality of heating modesbased on a function of the temperature rise, the engine load, and thestatus of the DPF.
 18. The method according to claim 8, furthercomprising turning the electric heater off when the engine load isgreater than approximately 25%.
 19. The method according to claim 8,wherein the step of operating the electric heater further comprises:generating electricity with a generator at low engine loads andutilizing the generated electricity to operate the electric heater. 20.The method according to claim 19, wherein the generator is driven by anengine that generates the exhaust gas that flows through the exhaustaftertreatment system.
 21. A method of heating an exhaust gas in anexhaust after treatment system comprising: selecting a heating modebetween a plurality of heating modes based on an engine load and astatus of a component of the exhaust aftertreatment system; heating theexhaust gas by operating the electric heater in the selected heatingmode; and operating the electric heater in a passive regenerationheating mode to heat an exhaust gas to a predetermined temperature toincrease NO₂ generation when an engine load is less than or equal toapproximately 25%.
 22. The method according to claim 21, wherein thestep of operating the electric heater further comprises: generatingelectricity with a generator at low engine loads and utilizing thegenerated electricity to operate the electric heater.
 23. The methodaccording to claim 21, further comprising operating the electric heaterto provide differential heating of the exhaust gas to the predeterminedtemperature.